Hydrodynamic Study of Cold Hydrodynamic Study of Cold Model Bubbling Fluidized Bed Model Bubbling Fluidized Bed System Using Bubble Caps System Using Bubble Caps

By Mandeep Sharma Dinanath Akela(Project Assistant, CMERI Durgapur) (Engineer, THERMAX Pune)

Central Mechanical Engineering Research Institute, Durgapur 713209, West Bengal, India

WHAT IS FLUIDIZED BED ? ?WHAT IS FLUIDIZED BED ? ?

Fluidized Bed is a system in which the air distributed by a grid or distribution

plate, is blown through the bed solids developing a “Fluidized Condition”,

which is formed when fine solid particles are transformed into a fluid like state

through contact with a gas or a liquid.

CHARACTERISTICS OF FLUIDIZED BEDCHARACTERISTICS OF FLUIDIZED BED

Fluidized beds display a number of liquid-like properties: Lighter objects float on top of the bed (i.e., objects less dense than the bulk density of the bed), The surface stays horizontal even in tilted beds, The solids can flow through an opening in the vessel just like a liquid, The beds have a “static” pressure head due to gravity, given by ρ0gh, Levels between two similar fluidized beds equalize their static pressure heads.

ADVANTAGES OF FLUIDIZED BEDADVANTAGES OF FLUIDIZED BED

Rapid mixing of solids, uniform temperature and concentrations. Applicable for large or small scale operations. Heat and mass transfer rates between gas and particles are high as compared to other modes of contacting. There is no moving part and hence a fluidized bed reactor is not mechanically agitated reactor. So, maintenance cost can be low. The reactor is mounted vertically and save space.

APPLICATIONSAPPLICATIONS

Reactors for Cracking hydrocarbons, Coal gasification, Carbonization, Calcination. Heat exchangers, Drying operations, Coating (example, metals with polymer) Solidification/Granulation, Adsorption/desorption, Bio-fuel generation

Parameters Affecting Fluidization Parameters Affecting Fluidization BehaviorBehavior

CLASSIFICATION OF FBC TECHNOLOGY…

Bubbling Fluidized BedBubbling Fluidized Bed Gas-solid bubbling fluidized bed (BFB) system is an efficient energy conversion method for combustion and gasification of solid fuels.

The combustion gas velocity is equal to “the minimum fluidization velocity”, bubbles are gas voids with very little or no solids, the upward flow rate for air/combustion gases is typically 2 - 3 m/s, the bed heights for BFBC plants are 0.5 to 1.5 m, gas residence time within the bed are between 1 and 2 seconds. Chemical and thermal behavior of BFB combustors is greatly influenced by the fundamental hydrodynamics of bed material, fluidizing medium and bubble behavior.

In actual BFB systems, it is very difficult to study these hydrodynamic parameters due to very high temperature and pressure conditions. Therefore, the objective of the present study is to investigate the hydrodynamic parameters by conducting experiments on laboratory scale BFB cold model at atmospheric conditions.

IMPORTANCE OF THE DISTRIBUTORIMPORTANCE OF THE DISTRIBUTOR

• With heavy load in large dia. beds, flat plates deflect unpredictably, hence curved plates (c & d) are used. These withstand heavy loads and thermal stresses well.• Type f consists of slits between grate bars, has the same characteristics as the flat perforated plate, but with a somewhat less uniform gas distribution.• Nozzle (type g) and bubble caps (type h) are widely used to prevent solids from falling through the distributor.

Different type of distributors have different significances:

EXPERIMENTAL EXPERIMENTAL SETUPSETUP

Min. Fluidization VelocityMin. Fluidization Velocity

The pressure drop through fixed beds of has been correlated by Ergun[13] using the equation:

ps

og

m

m

ps

og

m

mc d

u

d

ug

L

P

2

323

2 175.1

)(

)1(150

The minimum fluidization occurs when(drag force by upward moving gas ) = (weight of particles) or (P bed) (Area bed) = (Volume bed) (Fraction of solids) (specific weight of solids)

or

cgsmfmfbedbed g

gLAAP 1.

………………….. (1)

………………….. (2)

Umf , the superficial velocity of gas at minimum fluidization conditions, is found by combining eqn. (1) and eqn.(2):

)3......(....................

115075.12

3

32

2

3g

gsgp

g

gmfp

mfs

mf

g

gmfp

mfs

gdudud

The equation (3) can also be written as:

7.330408.07.33Re

2

1

2

32

g

gsgp

g

gmfpmf

gdud

…………………..(4)

21

3

4

dg

gspt C

gdu

21,Re

10

psphericaldC

p

gg

pgssphericalt d

dU

21

22

, 225

4

Terminal Velocity of Sand ParticlesTerminal Velocity of Sand ParticlesThe gas flow rate through a fluidized bed is limited on one hand by umf and on the other by entrainment of solids by the gas. When entrainment occurs these solids must be recycled or replaced by fresh material to maintain steady-state operations. This upper limit to the gas flow rate is approximated by the terminal or free-fall velocity of the particles [14], which can be estimated from fluid mechanics by:

Where Cd is the experimentally determined drag coefficient.

The expression for terminal velocity [14] is also given as below:

…..…(5)

…..…(7), 0.4<Rep<500

Terminal velocity(non-spherical),Ut’=Kt .Ut (spherical) ……... (8)

065.0log843.0 10

tK

5.0

88.431.53

4

g

vgpt

gdK For Re<1000,

, 0.4<Rep<500 …..…(6)

The correlation factor Kt is obtained by [15] :

Experimentally, terminal velocity of type sand I and sand II is find out by visual observations i.e. when the inlet air flow rate is increased beyond the flow rate at which entrainment of sand particles is started, the pressure drop across the bed is decreasing and at certain stage this pressure drop almost tend to zero when there is almost negligible sand material in the bed. At this stage, terminal velocity for both sand materials is obtained.

…... (9)

…... (10)

For Re <0.2,

Material Parameters Sand I Sand IISize Range, µm 150 – 300 212- 600Sand Mean Diameter, µm 233 518Particles Density,Kg/m3 2650 2650Bulk Density , Kg/m3 1431 1378Bed Voidage ε 0.46 0.47Min. Fluidization Velocity(Umf)

0.045 0.205

Terminal Velocity (Ut) 1.900 4.218Reynolds Number at min. fluidization

2.274 22.983

Reynolds Number at terminal velocity

27.440 135.407

Archimedes Number 1083 11904Geldart's Classification Group Group B Group B

Parameter Value

Viscosity of air, µg, (kg/m.s) 0.0000188

Diameter of fluidizer/vessel,

Dv, (m) 0.5

Height of fluidizer/vessel, Hv,

(m)1.5

Diameter of inlet air pipe, Dp,

(m)0.084

Diameter of bubble caps, Dbc,

(m)0.013

Height of bubble caps, Hbc,

(m)0.075

Number of bubble caps, Nbc 85

Geometric parameters Operating Parameters

Theoretical and Experimental Comparison of Umf & Ut for both sand materials

Bubble BehaviorBubble Behavior

4.35108.9

Re22.0

82.04

g

sg

gmbpmb

Ar

ArUd

pmbmb dKU

A.A. Minimum Bubbling Velocity (UMinimum Bubbling Velocity (Umbmb):):Fluidized systems with a small density difference between fluid and particles expand

uniformly, while those with a large density difference are generally unstable and produce bubbling. The lowest gas velocity at which bubbling occurs is called the minimum bubbling velocity. There are two correlations are presented to find umb:

1) The correlation to find umb is given by Broadhurst and Becker[16] :

2) Geldart[17] suggested a correlation for minimum bubbling velocity as:

Where Kmb is a constant whose value is 100 in cgs system. Davison & Harrison observed that the interval between minimum bubbling velocity and minimum fluidization velocity represents the stable uniform fluidization, which shrinks rapidly as the size of the particles increases.

…………… (11)

…………… (12)

B.B. Bubble sizeBubble sizeThere are several methods of observing bubbles in gas-solid fluidized beds: 1) Simple methods like visual observation if vessel is transparent, 2) still or cine photography, 3) using probes, 4) using two dimensional beds, 5) X-ray observation, 6) observation of particle movement, 7) observation of single isolated bubble.

The major advance in the study of fluid beds came with the investigations of single, isolated bubbles. The

Davison model remains useful as a first approximation. An isolated bubble in a fluid bed takes the form of an

indented sphere. The leading spherical surface is called the roof and the apex is the nose. The solids filling the

indentation are transported with the bubble for natural sand from 100 to 600 µm the wake fraction was from

0.22 to 0.28. The rise in velocity of an isolated bubble [18] depends on its size, given as:bbb gDgVU 711.079.0 2

1 …………… (13)

C. Bubble growth and coalescenceC. Bubble growth and coalescenceAfter detachment from the distributor, bubbles interact as they rise in the bed. The coalescence of a

pair ofbubbles has been investigated by Clift and Grace [19,20] and by Toei and Matsuno[21]. There are severalcharacteristics of the coalescence process. The leading bubble spreads horizontally as the trailing

bubbleelongates and accelerates into the wake of the leader. The volume of two bubbles increases by 20-

30%during coalescence, followed by a 10% decrease during consolidation after wake entry, giving a net

volumeincrease of 10-20%. There is increased gas flow through the emulsion phase between the bubbles.

Thefollowing relation finds the maximum size: 4.0

02

35.2

g

UUDD mfbedb …………… (14)

Bubble growth and splitting lead to a distribution of bubble size in the bed. Agarwal used a population balance to calculate the bubble distribution. Three correlations have been found to be reliable for mean bubble size.

15.............3.0exp0

bedbbbb D

HDDDHD

1) Mori & Wen [22] use the initial and maximum bubble sizes with first order growth rate (as it is the

most popular equation for calculation). With distributor plate having Bubble cap,

2.0

8.0

4.0 454.0

g

N

AHuu

HDor

bedmf

b

41

43

021

0

g

HHUUHD

mfb

2) Darton [23] has suggested another correlation for bubble size and the same is represented as:

3) Rowe [24] proposed a correlation to predict bubble size in a gas-solid fluidized bed (when size is not

restricted by the column dimension) as:

…………….. (16)

…………….. (17)

Ref. Darton [23] Mori [22] Rowe [24]

Sand I Sand II Sand I Sand II Sand I Sand IIUo 0.158 0.718 - - 0.158 0.718Umf 0.045 0.205 0.045 0.205 0.045 0.205H 0.1-1.5 0.1-1.5 0.1-1.5 0.1-1.5 0.1-1.5 0.1-1.5

Dbed 0.5 0.5 0.5 0.5 0.5 0.5Abed 0.196 0.196 0.196 0.196 0.196 0.196Nor 85 85 85 85 85 85

Dmax. 0.357 0.654 0.357 0.654 0.357 0.654Dmin. 0.080 0.147 0.080 0.147 0.080 0.147Db 0.053-

0.2170.098-0.399

0.096-0.244

0.176-0.448

0.052-0.267

0.142-0.588

Ub 0.514-1.038

0.697-1.406

0.691-1.101

0.936-1.491

0.509-1.151

0.838-1.708

Table . Comparisons between bubble size for both sand materials:

The experimental and theoretical maximum pressure drop across the bed at 100, 125, 150 mm fixed bed height conditions is 103, 139, 168 mm WC and 98.793, 136.85, 164.18 mm WC respectively for sand I and 44, 53, 65 mm WC and 40.098, 50.197, 66.642 mm WC respectively for sand II. The calculated umf for 0.233 mm mean diameter sand I is 0.045 m/s and for 0.518 mm mean diameter sand II is 0.205 m/s. The experimental value of umf varies from 0.05 to 0.06 m/s for sand I and for sand II, it varies from 0.206 to 0.209 m/s.

During the experiment, it was observed that the experimental value of Ut varies from 1.8 to 2.0 m/s for sand I and from 4.3 to 4.6 m/s for sand II. It is then compared with the theoretical value i.e.1.9 m/s and 4.218 m/s. Therefore, the ratio Ut/Umf for sand I is 30 - 33.3 and for sand II is 20.87 – 22.01 which shows the workability range of the present bubbling bed cold model.

Also it is observed that the bubble size varies from 0.080 m to 0.357 m for sand I, from 0.147m to 0.654 m for sand II at three bed heights i.e. 0.1m, 0.125m and 0.150m. It is observed from the graph that mean bubble size increases with increase in bed material height for both sand sizes. Also it is seen from the graphs that the mean bubble size for both sand sizes varies almost linearly with bubble rise velocity i.e. it increases when bubble rise velocity increase and vice versa. The bubble rise velocity varies from 0.514 to 1.708 m/s with superficial air velocity varies from 0.518m/s to 0.718m/s for both sand types.

Results & ConclusionResults & Conclusion

The hydrodynamics parameters like minimum fluidization velocity, minimum bubbling velocity, particle terminal velocity and the bubble size at different bed heights have been studied. The present study on cold model of bubbling fluidized bed with bubble caps distributor provides sufficient information on hydrodynamic parameters like minimum fluidization velocity, terminal velocity, the bubble size and bubble behavior in large beds, These parameters of a freely bubbling beds are needed to understand the contacting of gas and solids mixing.

Future WorkFuture WorkResults will be used in hot model

bubbling fluidized bed gasifier for generation of hydrogen from the biomass gasification of rice husk.

A CFD analysis will be done.